Molecular membranes, otherwise known as molecular sieves, are widely known in their use to separate mixtures of gas. Weakly branched molecular sieving silica membranes are of particular interest for use in fuel cells and petrochemical applications for their ability in selectively separating hydrogen and helium from gas mixtures. Environmental applications are also possible with these types of membranes to separate and enrich the methane (CH4) component from landfill gas and biogas. Ideally to fulfil these types of uses the membranes need to have both good pore size control and high permselectivity for hydrogen and helium, that is molecules with a kinetic diameter of less than 3.4 Å.
Sol-gel reaction process are widely employed to form molecular sieves. The use of sol-gel reaction processes are favoured for the formation of microporous dimensions, that is pore size less than 20 Å, in a molecular sieve. Brinker C. J. & Scherer G. W. (“Sol-Gel Science: the physics and chemistry of the sol gel processing”, Academic Press, San Diego, USA (1990)), outline that polymeric silicate gels are often synthesised by hydrolysing monomeric tetrafunctional alkoxide precursors, using a mineral acid (HCl) or a base (NH3) as a catalyst. The hydrolysis is then followed by the condensation reactions, an alcohol condensation and/or a water condensation. The resultant product is the formation of siloxan bonds (Si—O—Si), silanols (Si—OH), alcolhol and water, as outlined in the equations (1)-(3) below. The formation used in the sol-gel process must be finely tuned in order to control the pore size to molecular dimensions. The disadvantages of this process is that the matrix of the weakly branched silica film may be so dense that it results in no or very low permeation of gases.
Raman and Brinker, (“Organic template approach to molecular sieving silica membranes”. Journal of Membrane Science, 105, 273-279 (1995)), used tetraethylorthosilicate (TEOS) and methyltriethylsilane (MTES), absolute ethanol (EtOH), distilled water and HCl as the catalyst to produce intermediate film layers. Subsequently, they used several layers of non-hydrolysed TEOS to produce the top film. Raman and Brinker produced membranes with good poor size control for CO2 and CH4 separation, but with low permselectivity to He/CO2. The use of HCl as a catalyst in the preparation of the MTES/TEOS intermediate layer, Raman and Brinker does not provide for fine control over the pore size.
Kusakabe K., Sakamoto S., Sale T., and Morooka S., (“Pore structure of silica membranes formed by a sol-gel technique using tetraethytoxysilane and alkyltriethyloxysilanes” Sep. and Pur. Tech., 16, 139-146 (1999)), prepared templated membranes from TEOS and alkyltriethyloxysilianes. These membranes had reasonable pore size control and higher permeation but lower permselectivities for molecules with a kinetic diameters less that 3.4 Å.
De Vos R. M. and Verweij H. (“Improved performance of silica membranes for gas separation”, Journal of Membrane Science, 143, 37-51, (1998)) produced membranes using a single step catalysed hydrolysis sol-gel process and calcined at 400° C., with high permeancance to indicating that the permselectivity ability of these membranes is low for H2 and CO2 permeation. However when these membranes where calcined at 600° C. the permselectivity for H2 and CO2 increased by tenfold. The increase in permselectivity is mainly caused by pore reduction due to heat treatment but it also reduces permeation of gases.
Another technique for producing silica membranes is through the use of chemical vapour deposition (CVD) process as outlined by Lin C. L., Flowers D. L., and Liu P. K. T., (“Characterisation of ceramic membranes II. Modified commercial membranes with pore size under 40 Å”, Journal of Membrane Science, 92, 45-58, (1994)) and Wu J. C. S., Sabol H., Smith G. W., Flowers D. L. and Liu P. K. T., (“Characterisation of hydrogen-permselective microporous ceramic membranes”, Journal of Membrane Science, 96, 275-287, (1994)). Ha H. Y., Woo-Nam S., Hong S. A. and Lee W. K., (“Chemical vapour deposition of hydrogen-permselective silica film on porous glass support from tertraethylorthosilicate”, Journal of Membrane Science, 85, 279-290. (1993)) and Tsapatsis M. and Gavales G., (“Structure and aging characteristics of H2 permselective SiO2 Vycor membranes”, Journal of Membrane Science, 87, 282-296, (1994)) used chemical vapour deposition using vycor glass tubes as substrates. The membranes produced by the chemical vapour deposition have high permeance for H2/N2 and good separation capabilities for He/H2. Chemical vapour deposition requires considerably more equipment and may be more expensive than film dip coating processes.